Abnormal situations occurring due to failure of an electric line, failure of grounding, failure of a load, user error, etc. during use of an electric device for domestic or industrial purposes are exhibited through various types such as line short circuit and load over-current or arc.
Power off devices commonly called power outlets or sockets or multi strips or power boards do not have a function to sense an abnormal situation. Even when the power off devices have such a function, this function is only to cut off supply of electric power when several limited kinds of situations including, for example, leakage of electricity, are sensed. Electric leakage protection devices installed at electric lines in most buildings and houses only detect leakage of electricity. Thus, general power off devices often cannot detect abnormal situations other than leakage of electricity, and have a limitation in preventing electrical accidents.
Meanwhile, leakage of electricity is known as a cause of a fire. In practice, however, the possibility that a fire occurs due to leakage of electricity is low, even though the possibility that waste of electricity or electric shock occurs due to leakage of electricity is high. For this reason, power off devices, which only have a function to cut off supply of electricity through detection of leakage of electricity, have difficulty in providing an effect of preventing occurrence of a fire.
Rather, electrical fires may often occur due to spark ignition of arc caused by malfunction of an electric line or an electric appliance on an inflammable material therearound, overheat caused by overload, namely, overcurrent flowing through a load, or overheat generated when a terminal electric line connected to a load functions as a small capacity load under the condition that the terminal electric line is short-circuited, even though overcurrent does not flow through the load.
Meanwhile, although the possibility that a fire is generated due to arc is high, generation of arc is readily observed when a power switch is turned on, or a plug is connected or disconnected. For this reason, if supply of power is cut off whenever arc is detected, normal use of electric appliances may be difficult. Therefore, it is necessary to provide a power off circuit and a power off device, which can appropriately cut off supply of power by discriminating between short arc that may be generated when the user powers on or off and continuous arc that may generate a fire.